The present invention relates to the display of data (alpha-numerics, symbols and graphics) by use of an all-raster scan and more particularly to the superposition of data by modulating the all-raster scan of a cathode ray tube (CRT).
Video displays are now commonly used in connection with a wide variety of electronic instruments and systems including TVs, avionics equipment, word processing and computer displays, and a multitude of additional business and consumer equipment. In many video systems and particularly those similar to conventional TV systems using CRTs, the video displays are formed by the generation of an analog video signal which is in turn coupled to and synchronized with the raster scan of a CRT to control the intensity, and therefore the visual image, produced on the face of the CRT.
In some systems, visual images are displayed without the use of raster scan by a technique commonly known as stroke-writing. Stroke-writing employs a system wherein the deflection of an electron beam is moved about the face of a CRT much like the movement of a pencil to enable the continuous tracing of characters, symbols, or other information to be displayed. In this instance, the information is not generated as a series of intensity-modulated positions on the raster scan, but rather by a continuously moving and modulated electron beam defining the specific display patterns.
As might be expected, the technology has evolved even further resulting in hybrid systems, wherein the benefits of stroke-writing and raster scanning are combined. In such systems, video information is displayed during the raster scan and superimposed data is displayed by stroke-writing during the vertical retrace time of the raster scan. While such hybrid systems are highly desirable, the amount of information that can be displayed over the raster scan is significantly affected by the time of the vertical retrace. There is, therefore, a finite amount, and in various applications a too-restrictive amount, of information that can be displayed.
As will be appreciated, although stroke-written information tends to produce more visually acceptable displays, more power is required than that associated with conventional raster scans. Also, since raster scan techniques have long existed, many video systems are already equipped to display information by use of a raster scan. Accordingly, while stroke-written and raster techniques are highly developed, there has still been a continuing search for alternatives to stroke-written or hybrid displays.
One such technique includes the superposition of data by intensity modulating portions of the CRT during the raster scan to produce an all-raster CRT display. This system utilizes a predetermined memory space to store the information representing the data for each scan of the raster frame and to update that information for the next scan. The information stored in memory is used to control the intensity-modulation and form the superimpoed data. Such systems reduce the power required to produce superimposed data on an all-raster display but, in doing so, sacrifice some of the clarity normally associated with stroke-written data. However, in certain instances the reduced cost and power savings make such an all-raster system more desirable than any of the stroke-written or hybrid systems.
One of the drawbacks to an all-raster system displaying superimposed data is the memory space required to store the data so that it may be displayed during the raster scan. While a variety of techniques for storing data during a raster scan are known, as evidenced by reference to U.S. Pat. Nos. 3,787,819; 3,894,292; 4,052,719; and 4,011,556, there is still a need to reduce the memory space required for producing superimposed data. In particular, U.S. Pat. No. 3,787,819 describes a conventional system capable of generating data on a video monitor. In connecton with that display, a plurality of cyclic sub-memories are used equal in number to the maximum number of lines of text to be displayed on the video monitor. While this patent and the other referenced patents broadly describe the technology of the prior art, and in some cases work toward reducing the memory required in such systems, there is still a continuing need for other alternatives for reducing memory and thereby the cost of all-raster scanned systems.
Accordingly, the present system and techniques has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a reduction in memory required to produce superimposed data displays in all-raster scanned video systems.